This invention relates to an improved heterojunction bipolar transistor having a silicon-germanium base and a process of producing the transistor.
Recently interesting reports have been published about heterojunction bipolar transistors in which the emitter is formed of silicon and the base of a silicon-germanium alloy (herein expressed simply as SiGe) which is narrower in bandgap than silicon: e.g., IEEE IEDM Technical Digest (1990), pp. 17-20 and IEEE IEDM Technical Digest (1990), pp. 13-16. The use of SiGe for the base can enhance the efficiency of carrier injection from the emitter into the base, and in consequence current gain becomes sufficiently high even though the impurity concentration in the SiGe base is made higher than that in the conventional base silicon base by more than one order of magnitude. With a SiGe base, high performance at high frequencies can be realized by sufficiently raising the doping level in the base and reducing the base width. Furthermore, there is a possibility of shortening the base transit time of the carrier and consequentially further enhancing the high-frequency characteristics by grading the germanium profile in the SiGe base.
When conventional silicon homojunction bipolar transistors are operated at low temperatures near the liquid nitrogen temperature, current gain decreases to about 1/10 of that at room temperature. In contrast, heterojunction bipolar transistors having a SiGe base are very higher in emitter injection efficiency and current gain even at low temperatures and practically operatable at the liquid nitrogen temperature. Needless to mention, the important merit of the operation of semiconductor devices at the liquid nitrogen temperature is a great reduction in interconnection delay by reason of a decrease in interconnection resistance to 1/5 to 1/10 of that at room temperature. By incorporating SiGe base heterojunciton bipolar transistors in LSIs there is a good prospect of very high-speed operation of various circuits at low temperatures.
Meanwhile, much efforts have been devoted to the development of self-aligned bipolar transistors in which an emitter region and an external base region are formed in a self-aligned manner, with the intention of reducing the base capacitance and base resistance to thereby raise the operation speed of bipolar transistors. For example, new processes for the fabrication of self-aligned bipolar transistors having an epitaxially grown base layer are reported in IEEE IEDM (1990), pp. 603-606 and IEEE IEDM (1990), pp. 607-610. However, the application of these processes to the fabrication of a SiGe base heterojunction bipolar transistor is problematic for several reasons. It is difficult to suitably differentiate the doping level in an external base from that in an intrinsic base, and a high-temperature heat treatment employed for the formation of the emitter adversely affects the characteristics of the base. (The problems will be explained hereinafter more particularly by comparison with the present invention.)